A spark plug is attached to a combustion apparatus (e.g., an internal combustion engine), and is employed for ignition of an air-fuel mixture or the like. In general, the spark plug includes an insulator having an axial hole; a center electrode inserted into a forward end portion of the axial hole; a terminal electrode inserted into a rear end portion of the axial hole; and a metallic shell provided around the insulator. A resistor may be provided within the axial hole and between the center electrode and the terminal electrode for reducing radio noise generated in association with operation of the combustion apparatus (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-66086).
Generally, the resistor is formed by charging, into the axial hole, a resistor composition containing, for example, glass powder (containing silicon dioxide (SiO2) and boron oxide (B2O3)), an electrically conductive material (e.g., carbon black), and ceramic particles, and by heating and compressing the resistor composition through hot-pressing of the terminal electrode toward the center electrode. The thus-formed resistor is in a phase-separated state such that an intervening phase containing a relatively large amount of B2O3 is present around aggregate phase containing a relatively large amount of SiO2. The aggregate phase is composed of glass grains from which a B2O3-rich glass component has been melted, and the intervening phase is generally composed of a molten B2O3-rich glass component. The intervening phase contains the electrically conductive material and ceramic grains. Thus, the center electrode is electrically connected to the terminal electrode via electrically conductive paths included in the intervening phase, the paths being formed of the electrically conductive material.
From the viewpoint of improving the effect of preventing radio noise (hereinafter may be referred to as “radio-noise-preventing effect”), desirably, the distance between the center electrode and the terminal electrode in the direction of the axial line is increased; i.e., the length of the resistor is increased. However, when a resistor composition containing the aforementioned glass powder having a relatively large mean particle size is employed, and the distance between the center electrode and the terminal electrode is made relatively large, difficulty is encountered in sufficiently increasing the density of the resistor, for the following reasons.
Specifically, since the glass powder having a large mean particle size is less likely to be melted during heating (i.e., a small amount of B2O3-rich glass component is melted from glass particles), gaps between aggregate phase are insufficiently filled with an intervening phase, and voids (pores) are generated between the aggregate phase. Thus, pressure is likely to be lost during compression. When the distance between the center electrode and the terminal electrode is relatively small, pressure loss is not increased greatly, and thus sufficiently large pressure can be applied to a forward end portion (a portion away from the terminal electrode) of the resistor composition. Therefore, voids (pores) between the aggregate phase can be eliminated through compression in the entire resistor. Consequently, gaps between the aggregate phase are filled with the intervening phase, and the density of the resistor can be sufficiently increased.
Meanwhile, when the distance between the center electrode and the terminal electrode is relatively large, pressure loss during compression is increased, and pressure applied to a forward end portion of the resistor composition is reduced. Therefore, voids generated between the aggregate phase remain in a forward end portion of the resistor; i.e., the density of the resistor is lowered. In this case, the lower the density of the resistor, the smaller the number of electrically conductive paths in the resistor. Thus, the resistance of the resistor having a low density may be drastically increased through partial oxidation of the electrically conductive paths during use of the spark plug, resulting in deterioration of load life performance.
When glass powder having a small mean particle size (e.g., about 100 μm) and being likely to be melted is employed for increasing the density of the resistor, a larger amount of a B2O3-rich glass component may be melted from glass particles, and gaps between the aggregate phase may be more reliably filled with the intervening phase. However, in such a case, an amount of the B2O3-rich glass component, which has a relatively low viscosity, is increased in the glass material melted through heating, and the viscosity of the molten glass material is lowered (i.e., the viscosity becomes nearly equal to that of water). Therefore, when pressure is applied to the resistor composition, a larger amount of the glass material is likely to enter a gap between the outer wall of the terminal electrode and the inner wall of the axial hole, and the aforementioned voids (pores) may be insufficiently eliminated through compression. Consequently, the density of the resistor may be lowered, resulting in unsatisfactory load life performance.
Meanwhile, when the resistor composition is prepared by uniformly mixing glass powder having a relatively large mean particle size with glass powder having a relatively small mean particle size, a reduction in viscosity of the molten glass material during heating may be prevented while gaps between the aggregate phase are filled with the intervening phase. However, in such a case, there occurs a phenomenon that glass particles having a relatively small mean particle size are aggregated together. Therefore, although gaps between the aggregate phase are filled with the intervening phase in a portion of the resistor, voids are generated between the aggregate phase in the remaining portion of the resistor, as in the case of employment of only glass powder having a relatively large mean particle size. Consequently, the density of the resistor may fail to be increased, resulting in unsatisfactory load life performance.
In view of the foregoing, an advantage of the present invention is an increase in the density of a resistor that provides excellent load life performance in a spark plug in which the distance between the forward end of a terminal electrode and the rear end of a center electrode is relatively large.